The wide variety of industrial applications each independently calls for a level of robustness from the physical (PHY) layer, all the way up to applications. Time sensitive networking (TSN) algorithms are paramount to industrial automation and process control, while resistance to harsh chemicals is critical in mining, chemical, and petrochemical industries.

Industrial Ethernet has been increasingly adopted in manufacturing applications for its streamlined structure and cost-effectiveness. The IP-based protocols allow industries to benefit from economies of scale and maximized throughput, reliability, determinism, and Quality of Service (QoS).

In a recent HMS report, industrial Ethernet now accounts for 52% of the industrial interconnect market, while Fieldbus technology is now at 48%. Many motion control products used in manufacturing, automation, and process and control applications are already equipped with RJ45 ports, allowing them to be monitored and controlled in an Ethernet network. The USB3 vision and GigE vision standards are major contenders in factory vision applications.

With the myriad of cable options and builds, data cables must be tailor-made for each individual application, depending on the environment, hardware interface, and cost-effectiveness.

Ethernet Cabling in Motion Control

While industrial wireless sensor networks (IWSN) are proliferating in applications monitoring and tracking applications, their associated latencies (10-100 ms) are not low enough to support automation applications where many Ethernet-based TSN protocols have already achieved response times of 1 ms with less than 1 ms of jitter. Although it may be desirable to have automation and robotics controlled remotely, there is no guarantee of QoS with bit error rates (BER) as high as 10-2, while Ethernet protocols can achieve a low BER of 10-12, allowing for the ability to run millions of cycles without mishap. Still, the PHY layer relies heavily on the integrity of its interconnect, so the design of the Ethernet cable must be robust enough to withstand the constant flexure.

Modern industrial robotics demand more flexibility in limited working environments, forcing a trend towards more compact six-axis robot arms (Figure 1). Traditionally, cables are clamped down to the robot and extend between robot parts normal to a pivotal axis so that the effective length does not change. Newer robots not only contain multiple pivot points for bending, but rotational joints for twisting and turning the machine. The length of the cable needed for the variety of axial and rotational adjustments varies, rendering ineffective the legacy solution of fixing cables down, and potentially accelerating cable degradation.

The increase in complexity of machine motions necessitates the use of highly flexible data, fiber, Ethernet, bus, motor, and servo cables. Cables in these applications not only undergo constant bending, but also extreme torsional and longitudinal stresses. This kind of stress can rapidly deteriorate the cable jacketing, shielding, and wire bundles within the cable. Cable carriers external to the robot arm and cable routing structures inside the robot arm help minimize the stresses on the hoses and cables. Still, it is necessary to implement cabling with a high number of flex cycles and abrasion resistance for a more reliable connection.

Comparison of USB 3.0 and GigE Vision

Several cable failures can occur, including shielding unraveling, conductor tearing or breaking, or a failure at the junction point between the connector and cable. The high-vibration environment for cables on robotics can cause the traditional RJ45 Ethernet connectors to come loose, potentially causing failures. The constant stress at the fulcrum point between the connector and cable can also degrade, leading to an intermittent connection. Alternatives such as M8 and M12 connector heads screw down to avoid becoming dislodged in motion applications. These connectors are often IP67- or IP68-rated for protection from fine particulates such as dust, as well as protection from temporary liquid immersion, which is helpful in humid environments where moisture ingress can not only damage cable assemblies, but sensitive connected circuitry.

The frictional forces on the cable under constant flexure cause heat and ultimately tearing within the cable. Furthermore, cables bunched together tend to rub against each other, potentially damaging the cable jacketing. To mitigate these effects, certain metal alloys are employed to increase flexibility. Cable jackets and insulation material such as polyurethane (PUR) tend to have a high tensile strength (flexibility) and resistance to tearing/abrasion. Highly flexible cables must specify the flex rating to properly assess its practicality in an automation application; for example, the 1-million-cycle test at a minimum bend radius at ten times the cable outer diameter (~50 mm). While it is critical that the cabling be robustly designed, modern machines used in factory automation must be able to see and assess their surroundings with vision processors and software algorithms.

Vision Applications

Recent technological advancements for high-accuracy, high-bandwidth, and high-frame-rate imaging, including smart cameras and CMOS image sensors, contribute to the overall trend of higher-throughput vision applications. Long-range fixed or mobile observation that offers real-time HD image capture in harsh conditions for industrial or military use requires ruggedized cable solutions. In the switch from analog systems to digital, there are two main interfaces that seem to be leading the way: GigE vision or USB 3.0 (see table).

PC-based vision systems will come equipped with USB 3.0 ports, making them a natural option, while smart cameras may require additional infrastructure, marginally increasing the investment for a smart camera system. Another apparent differentiating factor is bandwidth: USB 3.0 offers more than 300 Mbps of speed while GigE vision is more than 100 Mbps. While this can be a major deciding factor for the choice of imaging technology, there are other factors where GigE vision reigns; for example, GigE cables run 100 meters without compromising data transfer, whereas USB 3.0 is only 5 meters. Longer cable lengths for USB vision can be accomplished through the use of repeaters and equalizers in active cabling.

Figure 2 - GigE cable with rugged RJ45 connectors outfitted with thumbscrews in order to prevent the cable from getting dislodged in high-movement applications such as factory automation.

In motion control and automation applications, a simple video capture card can be added to the PC tower in the vicinity, allowing for low-cost and scalable multi-camera architecture with relatively high processing capabilities. GigE vision can still offer a more modular camera system as each smart camera has its own independent image processing without the need for peripheral devices such as PC towers or frame-capture cards. This allows for these independent vision stations to fit into smaller dimensions on the plant floor. As shown in Figure 2, connector heads come fitted with thumbscrews to keep the connection from becoming loose for motion applications that expose the cabling to excessive vibrations. While the application is a major deciding factor of the data cable interface, the environment in which the cable resides determines the level ruggedness, and therefore design complexity required for a connection to thrive over its lifetime.

Harsh Cable Environments

A cable can endure a host of environments in industrial applications. In oil refineries, cables are likely to interact with oil, while cables running in chemical facilities may need strengthening against highly aggressive liquids. Marine applications require protection from moisture ingress and corrosive salts in the atmosphere, particularly in saltwater environments. Agricultural cables may need protection from the constant bombardment of UV rays from the Sun that ultimately shorten the lifespan of the cable. The cable jacket and connector construction is the primary defense for the various environmental agitators. Thermoplastics or thermosets are the two materials that generally are leveraged in cable jackets to ensure a certain degree of ruggedness.

Thermoplastics such as polyvinyl chloride (PVC) and polyethylene (PE) are typically much more simple to manufacture and alter than thermosets; cable jackets can be made en masse through the extrusion process, and are much more readily stripped for soldering on connector heads. Thermosets such as neoprene or polyurethane (PUR) are permanently set after the curing process, making them much more rigid and able to handle extreme temperatures without the risk of deforming. In many cases, these polymers often require additives such as plasticizers to enhance or tailor their properties for low-temperature flexibility, impact and abrasion resistance, and resistance to oils or chemicals; for example, PVC on its own has nominal flame-retardant properties, but plenum PVC (P-PVC) is far more flameproof.

In most cases, plasticizers are added to increase the viscosity of the material to generate more cable flexibility and durability. Cables without resistance to chemicals in a highly aggressive chemical atmosphere will eventually fail, even in storage. The plasticizers in the material seep out due to chemical or oil ingress swelling, deforming, and even cracking the cable jacket.

Conclusion

The interconnected backbone for factory automation and motion control must be designed to cope with heavy vibrational stressors. High-flex-cycle cabling is essential for a reliable connection for these devices that have some of the most stringent timing and jitter requirements. Furthermore, outdoor environments will expose an interconnect to stressors such as ozone, oxidation, UV rays, and moisture, while factories with heavy chemical utilizations can rapidly deteriorate a cable. The cable jackets must then provide adequate protection from the environmental agitators to extend the life of cable, and minimize the cost and maintenance that comes with repairing or replacing damaged cabling.

This article was written by Dustin Guttadauro, Product Manager at L-com Global Connectivity, North Andover, MA. For more information, visit here.

Articles: Test & Measurement

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